RTCP Packet Format

RTCP is the Real-time Transport Control Protocol, which may be used as a lightweight companion to RTP to convey a number of statistics and other information about an RTP flow between recipients and senders.

The header is illustrated below.

 

Table 5.3: The Real Time Control Protocol

SR or RR

: The first RTCP packet in the compound packet must always be a report packet to facilitate header validation as described in Appendix A.2. This is true even if no data has been sent nor received, in which case an empty RR is sent, and even if the only other RTCP packet in the compound packet is a BYE.

Additional RRs

: If the number of sources for which reception statistics are being reported exceeds 31, the number that will fit into one SR or RR packet, then additional RR packets should follow the initial report packet.

SDES

: An SDES packet containing a CNAME item must be included in each compound RTCP packet. Other source description items may optionally be included if required by a particular application, subject to bandwidth constraints (see Section 6.2.2).

BYE or APP

: Other RTCP packet types, including those yet to be defined, may follow in any order, except that BYE should be the last packet sent with a given SSRC/CSRC. Packet types may appear more than once.